Method for forming information carrying signals in an electrical power supply network

ABSTRACT

A method for forming and transmitting information carrying signals in an electrical power distribution network is disclosed. A step-shaped current signal is produced by switching at least two purely ohmic loads between a pair of power lines. The ohmic loads are switched between the power lines in accordance with a predetermined sequence so that a current signal having the desired waveform results. The step-shaped information carrying signal usually has a frequency which is higher than the power transmission frequency of the supply network.

FIELD OF INVENTION

This invention relates to a method for forming and transmittinginformation carrying signals in an electrical power supply network.

BACKGROUND OF THE INVENTION

The primary use of an electrical power supply network is to distributeenergy to consumers. Notwithstanding this primary purpose, an electricalpower distribution network can be used with advantage to transmitfrequencies other than the fundamental power frequency for datapurposes.

One system for accomplishing such data transmission is disclosed in anarticle entitled "Data Transmission over Distribution Systems" by A. J.Baggott, Electrical Times Mar. 12, 1970. The power transmissionfrequency of the distribution network is used as the carrier waveform.Information signals are encoded on the carrier by selectively distortingthe carrier waveform.

The distortions of the carrier waveform are formed by using a thyristorto switch a resistive, capacitative, or inductive load between two powerlines. The system for transmitting data described in the aforementionedarticle is particularly adapted to transmit such data in the directionof energy flow in the power supply network.

A similar system for the transmission of data in an electrical supplynetwork is disclosed in Swiss Patent No. 404,775. In the system of SwissPatent No. 404,775, data is transmitted in a direction counter to thatof the energy flow. The data carrying signals are generated by switchinga capacitative load between two power lines.

It is an object of the present invention to provide an improved methodfor forming and transmitting information carrying signals in an electricpower distribution network, which method would:

(a) involve the use of simple and inexpensive electrical components,

(b) produce information carrying signals of low harmonic content, sothat requirements relating to cross-talk are easily met, and

(c) offer improved energy exploitation, and

(d) economize costs.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the invention, aninformation carrying signal is formed in an electrical power supplynetwork by sequentially switching at least two purely ohmic loadsbetween a pair of power lines. The loads are switched in accordance witha predetermined sequence under the control of a microcomputer or othercontrol logic circuitry so as to generate a step-shaped informationcarrying current signal. Preferably, the information carrying currentsignal is formed during one-half cycle of a power transmission waveformof the electrical power distribution network. The step-shaped currentsignal generally has a frequency which is significantly higher than thefrequency of the power transmission waveform.

The above-described method enables the production of current signalswithin the 5 kilohertz to 15 kilohertz range. Such current signalseasily fit within one half-cycle of the power transmission frequency,which frequency is generally in the 50-60 hertz range. Such currentsignals are suitable for transferring information in a direction counterto the direction of energy flow in the network. Information may betransferred from a low tension part of the network to a high tensionpart of the network by means of transformers arranged between the lowtension and high tension portions of the network. Illustratively, thecurrent signals produced in accordance with the inventive methodapproximate sine curves, thereby resulting in improved energyexploitation. Finally, the current signals have a reduced harmoniccontent so that problems resulting from cross talk with other parts ofthe network are minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an apparatus for originatinginformation carrying signals in an electrical supply network bysequentially switching a plurality of resistances between two powerlines in accordance with an illustrative embodiment of the invention.

FIG. 2 schematically illustrates a sequence for switching two of theresistances of FIG. 1 and the resulting current signal,

FIGS. 3a through 3c illustrate alternative sequences for switching twoof the resistances of FIG. 1 to produce the current signal of FIG. 2.

FIG. 4 illustrates the type of signal current which may be produced whenmore than two resistances are sequentially switched between the powerlines of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, an apparatus 1 for generating informationcarrying signals in an electrical power supply network comprisessubstantially pure ohmic resistances R₁, R₂ through R_(N). Theresistances R₁ through R_(N) are directly connected to a first powerline, for example a phase line, designated Ph. The resistances R₁through R_(N) are also connected to a second power line by means ofswitches S₁, S₂ through S_(N), respectively. The second power line maybe a neutral line which is designated Mp in FIG. 2.

The control logic circuitry 2 determines the time intervals during whicheach of the individual switches S₁ through S_(N) is closed and,consequently, the time intervals during which each of the individualresistances R₁ through R_(N) is switched between the power linesdesignated Ph and Mp. Preferably the control logic 2 is a microcomputeralthough other types of control logic circuitry known to those skilledin the art may also be used. At any particular instant in time, theinformation carrying current signal I produced by the sequential closingand opening of switches S₁, S₂ through S_(N) is the sum of theindividual currents I₁, I₂ through I_(N) passing through the individualresistances R₁, R₂ through R_(N).

FIG. 2 shows the step-shaped current signal I (solid step-shaped curve)which is produced when two resistances R₁ and R₂ are switched by meansof switches S₁ and S₂ between the power lines Ph, Mp in accordance witha particular time sequence. As previously indicated, the opening andclosing of switches S₁ and S₂ is under the control of logic circuitry 2.The total current signal I is determined at any time by the addition ofthe partial currents I₁ and I₂ flowing through the resistances R₁ and R₂respectively. One oscillation of current signal I has a period T, whichperiod T extends from the time designated -6t/6 to the time designated+6t/6 in FIG. 2. It is assumed that the resistances R₁, R₂ are in aratio of 2:1.

The switching sequence for the switches S₁ and S₂ is illustrated at thebottom of FIG. 2. The switches S₁ and S₂ are open during the timeintervals indicated by the level "0" and are closed during the timeintervals indicated by the level L.

Thus, in the time interval from -6t/6 to -5t/6 switch S₁ is closed andswitch S₂ is open so that a current I₁ of magnitude A₁ flows throughresistance R₁ and no current flows through resistance R₂. In theinterval from -5t/6 to -t/6, switch S₁ is open and switch S₂ is closedso that resistance R₂ is connected between the power lines Ph, Mp.Accordingly, during the interval -5t/6 to -t/6, a current I₂ ofmagnitude A₁ +A₂ flows between the power lines Ph and M_(p). Theresistance R₂ draws a larger current than the resistance R₁ becauseresistance R₂ is smaller by a factor of two. In the interval from -t/6to +t/6, S₁ is closed and S₂ is open so that a current I₁ of magnitudeA₁ flows through resistance R₁. Similarly, in the interval between +t/6and +5t/6, both S₁ and S₂ are open so that neither of the resistancesR₁, R₂ is connected between the power lines Ph, Mp and the partialcurrents I₁ and I₂ are both zero. Finally, during the interval from+5t/6 to +6t/6 the switch S₁ is closed so that a current I₁ of magnitudeA₁ flows through resistance R₁.

As can be seen from FIG. 2, one oscillation of the currently signal Icomprises two oppositely directed half oscillations extending from -6t/6to t_(o) to and from t_(o) to +6t/6, respectively. The two-step,step-shaped current signal I closely approximates a sine curve of periodT=12t/6, which curve is shown in FIG. 2 as a dotted line.

During one period of oscillation T of the current signal I, theresistance R₁ is switched between the power lines Ph, Mp for an amountof time equal to T/3 and the resistance R₂ is switched between the powerlines Ph, Mp for an amount of time equal to T/3. During a further amountof time equal to T/3, both switches S₁ and S₂ are open and neitherresistance is switched between the power lines Mp, Ph.

As an alternative to using resistances R₁, R₂, which are in a ratio of2:1, other values of the resistances R₁, R₂ may be used to generate thecurrent signal I of FIG. 1. The switching sequence of switches S₁, S₂should, of course, be chosen based on the values of resistances R₁, R₂.For example, the current signal I of FIG. 2 can be produced using theswitching sequences of FIGS. 3a, 3b, 3c when the resistances R₁, R₂ areequal in size.

In each of the FIGS. 3a, 3b and 3c, the state of the switches S₁ and S₂has been illustrated, wherein 0 represents an open switch S₁, S₂ and L aclosed switch S₁, S₂.

In FIG. 3a, the oscillation period T of the current signal I extendsfrom the time designated t₁ to the time designated t₇. Switch S₁ isclosed during the interval from t₁ to t₅, during which intervalresistance R₁ is switched between the power lines Ph and Mp. Switch S₂is closed during the intervals t₂ to t₄, during which interval theresistance R₂ is switched between the power lines Ph and Mp. Switch S₁is open from t₅ to t₇ and switch S₂ is open from t₁ to t₂ and from t₄ tot₇. The two switches S₁ and S₂ are both closed simultaneously during theinterval from t₂ to t₄. As can be seen from FIG. 3a, the switch S₁ isclosed for two-thirds of the period T, while the switch S₂ is closed forone-third of the period T. Thus, resistances R₁ and R₂ are unevenlyloaded in view of the different lengths of time during which theswitches S₁, S₂ are closed.

To load the resistances R₁, R₂ evenly, one may use the switchingsequence of FIG. 3b. During a first period T of the current signal I,the switch S₁ will be closed from t₁ to t₅ and the switch S will beclosed during t₂ to t₄. In the following period T, switch S₁ will beclosed only from t₈ to t₁₀, but switch S₂ will be closed from t₇ to t₁₁.This alternating sequence repeats itself in the following periods T.

In the switching program illustrated in FIG. 3c, the resistances R₁, R₂are also equally loaded. In particular, the switches S₁, S₂ are closedfor equally long intervals. Howevers, as shown in FIG. 3c, the intervalsduring which switch S₁ is closed do not entirely overlap the equallylong intervals during which switch S₂ is closed. The closing intervalsof switch S₂ are displaced from the closing intervals of switch S₁ byone-third the duration of the intervals. Thus, switch S₁ is closed fromt₁ to t₄, t₇ to t₁₀ and so on. Switch S₂, by contrast, is closed from t₂to t₅, t₈ to t₁₁ and so forth. Both switches S₁, S₂ are open at the sametime during the periods t₅ to t₇, t₁₁ to t₁₃ and so forth.

Preferably, the frequency of the information carrying current signaloscillation of FIG. 2 is independent of the power transmission frequencyof the power supply network or any harmonic thereof. Typically, thefrequency of the current signal oscillation I is in the range of 5 kHzto 15 kHz. This is considerably higher than the power transmissionfrequency of the supply network, which is typically in the range of 50Hz to 60 Hz. Thus, a single oscillation of the current signal I caneasily fit within a half oscillation of a power transmission waveform ofthe supply network. The current signal I can be formed during one orboth half oscillations of the power transmission waveform. Detection ofsuch current signals may be accomplished by conventional means known tothose skilled in the art.

As indicated above, the current signal I shown in FIG. 2 has a waveformwhich approximates a sine curve. For this reason, the energy utilizationof the two-step, step-shaped current signal of FIG. 2 is improved incomparison to that of a purely rectangular oscillation. The currentsignal I is of low harmonic content. Any higher harmonics contained inthe current signals have relatively low amplitudes so that cross talkproblems are limited. Furthermore, the current signal I may be formed ina low-tension part of the network and transferred by way of transformersto a higher tension part of the network.

In a particular embodiment of the invention, it may be desirable tosequentially produce several current signals, wherein each successivecurrent signal has a different frequency corresponding to a differenttype of information. Illustratively, a second current signal may beformed by employing resistances R₃, R₄ (not shown). The resistances R₃,R₄ may be similar in size to or larger that the resistances R₁, R₂. Theresistances R₃, R₄ are sequentially switched between the power lines Ph,Mp by means of switches S₃, S₄ (not shown). Illustratively, the controllogic circuitry 2 is programmed so that the switching sequence ofswitches S₃ and S₄ produces a second current signal of a frequencydifferent from that of the first current signal which results from theswitching sequence of switches S₁ and S₂. Detection is simplified whenthe first current signal (produced by switches S₁, S₂) is not in aharmonic relationship with the second current signal (produced byswitches S₃, S₄).

Approximation of the current signal I to the sine shape can be furtherimproved if more than two resistances are used. FIG. 4 shows a half-waveof the step-shaped flow of current I, when six resistances R₁ through R₆are sequentially switched between power lines Ph, Mp by way of switchesS₁ through S₆. The ideal sine waveform is illustrated in FIG. 4 as adotted line. Near the bottom of FIG. 4, the switching sequence ofswitches S₁ to S₆ has been schematically illustrated. The level "0"represents an interval in which a switch is open. The level "L"represents an interval during which a switch is closed. As can be seenin FIG. 4, the switches S₁ through S₆ are closed for unequal intervalsof time. This is provided for in the control logic circuit 2 of FIG. 1.For example, if the control logic circuit 2 is a microcomputer, theintervals during which the switches S₁ through S₆ are closed aredetermined by a program stored in the microcomputer. Such programming isknown to those skilled in the art and need not be further explainedhere. The total current I flowing through all the resistances R₁ to R₆at any given moment is obtained by adding the individual partialcurrents flowing through each of the resistances R₁ to R₆ at the givenmoment.

In an alternative embodiment of the invention, a current signal I ofsimilar shape to that illustrated in FIG. 4 can be produced by means ofresistances R₁ through R_(N). The resistances R₁ through R_(N) aresequentially switched between the power lines Ph, Mp by switches S₁through S_(N) for equal intervals. This may be accomplished inparticular cases by reprogramming the control logic circuit 2.Advantageously, the resistances and intervals may be arranged so that adigital to analog conversion can occur.

Finally, the above described embodiments of the invention are intendedto be illustrative only. Numerous alternative embodiments may be devisedby those skilled in the art without departing from the from the spiritand scope of the following claims.

I claim:
 1. A method for forming an information carrying signal,comprising a group of step-shaped, approximated sine curves, in analternating current electrical power supply distribution network, saidmethod comprising the steps of sequentially switching at least twosubstantially pure ohmic loads between a pair of power lines of saidnetwork, said loads being switched under the control of control logiccircuitry in accordance with a pre-determined sequence during at leastone-half cycle of a power transmission waveform of said network togenerated a step-shaped current signal whose frequency is substantiallyhigher than the frequency of said power transmission waveform.
 2. Themethod of claim 1 wherein said at least two substantially pure ohmicloads comprise first and second resistances R₁ and R₂, said resistancesR₁, R₂ being sequentially switched between said pair of power lines byway of first and second switches S₁ and S₂ respectively.
 3. The methodof claim 2 wherein said resistances R₁ and R₂ are in a ratio of about2:1.
 4. The method of claim 3 wherein said stepped-shaped current signalhas a predetermined period of oscillation T and wherein during eachperiod T, said first switch S₁ is closed for an amount of time T/3, saidsecond switch is closed for an amount of time T/3, and both switches S₁,S₂ are simultaneously open for an amount of time T/3.
 5. The method ofclaim 4 wherein said current signal I is the sum of a first partialcurrent I₁ which flows through said first resistance R₁ and a secondpartial current I₂ which flows through said second resistance R₂, andwherein during each period T said first partial current flows for anamount of time T/3, said second partial current flows for an amount oftime T/3 and neither partial current I₁, I₂ flows for an amount of timeT/3.
 6. The method of claim 2 wherein said first and second resistancesR₁, R₂ are of substantially equal size.
 7. The method of claim 6 whereinsaid step-shaped current signal has a predetermined period ofoscillation T and wherein during each period T said first switch S₁ isclosed for substantially twice as much time as said second switch S₂. 8.The method of claim 6 wherein said step-shaped current signal enduresfor at least two periods of oscillation, each period of oscillationhaving a predetermined duration T and wherein during said first periodof oscillation said first switch is closed for substantially twice asmuch time as said second switch and during said second period ofoscillation said second switch is closed for substantially twice as muchtime as said first switch.
 9. The method of claim 6 wherein saidstep-shaped current signal has a predetermined period of oscillation Tand wherein during each period T said switches S₁, S₂ are closed forequally long but only partially overlapping intervals of time.
 10. Themethod of claim 1 wherein N resistances R₁ to R_(N) are sequentiallyswitched between said power lines by means of corresponding switches S₁to S_(N), N being greater than
 2. 11. The method of claim 10 whereininformation carrying signals of more than one frequency are produced,each of said frequencies corresponding to a different type ofinformation.
 12. A method for forming an information carrying signal, inan alternating power supply distribution network, said method comprisingthe steps of sequentially switching at least two substantially pureohmic loads between a pair of power lines of said network, said loadsbeing switched under the control of control logic circuitry inaccordance with a predetermined sequence during at least one-half cycleof a power transmission waveform of said network to generate at leastone step-shaped signal approximating a sine curve, thereby generatingsaid information carrying signal, and transmitting said informationcarrying signal through the network in a direction opposite to thedirection of power flow in the network.
 13. A method for forming aninformation carrying signal, comprising at least one step-shaped,approximate sine curve, in an alternating current electrical powersupply distribution network, said method comprising the steps ofsequentially switching at least two substantially pure ohmic loadsbetween a pair of power lines of said network, said loads being switchedunder the control of control logic circuitry in accordance with apre-determined sequence during at least one-half cycle of a powertransmission waveform of said network to generate a step-shaped currentsignal whose frequency is substantially higher than the frequency ofsaid power transmission waveform.
 14. A method for forming informationcarrying signals in an alterniating current electrical powerdistribution network, said method comprising the steps of sequentiallyswitching a plurality of substantially pure ohmic loads arranged ingroups between a pair of power lines of said network, said loads beingswitched under the control of control logic circuitry in accordeancewith a predetermined sequence during at least one-half cycle of a powertransmission waveform of said network to generate a group of step shapedapproximately sine shaped information carrying signals, havingfrequencies substantially higher than the frequency of said powertransmission waveform.
 15. The method of claim 14, wherein said stepshaped approximately sine shaped information carrying signals aretransmitted in a direction opposite to that of the direction of powerflow in said network.
 16. The method of claim 14, wherein said powerlines are a phase lead and a neutral wire.